The present invention generally relates to a liquid crystal displaying apparatus for effecting an image displaying operation and a projection displaying apparatus using it, and a view finder apparatus.
In order to obtain larger picture face images, a method is known of forming optical images corresponding to image signals on a light valve, irradiating light to the optical images, and effecting an enlarged projecting operation on a screen by a projection lens. Lately, a projection displaying apparatus using a liquid crystal panel as a light valve has been used. Although there are many types of liquid crystal panels, twisted nematic (TN) liquid crystals are used as liquid crystals so as to obtain high picture quality of projecting images. A thin film transistor element (TFT) provided on each picture element is generally used as a switching element.
Basic construction of projection display apparatus using a TN liquid crystal panel is shown in FIG. 18. An outgoing light from a light source 11 is inputted into a projection lens 15 after being transmitted through a TN liquid crystal panel 14. The TN liquid crystal panel 14 forms optical images as changes in transmission index in accordance with image signals. Optical images formed on the TN liquid crystal panel 14 are enlarged and projected onto a screen 16 by the projection lens 15.
The TN liquid crystal panel 14 is composed of a liquid crystal cell 20 with a TN liquid crystal 19 being charged between two substrates 17 and 18, and two polarizing plates 21 and 22 to be arranged on both sides thereof. A picture element electrode, a TFT, a gate wire and a source signal wire are formed on one substratum 18. An opposite electrode is formed on the other substrate 17. A black matrix for shielding the light is formed on the TFT, the gate wire and the source signal wire. Alignment films are formed respectively on the picture element electrode and the opposite electrode, and crystal molecular long axis of the TN liquid crystal 19 becomes twisted by 90.degree. by the rubbing processing of the alignment film. A polarized light axis of the polarizing plate 21 on the incident side is parallel or perpendicular to a long axis direction of the liquid crystal molecule which is in contact with a substratum 18 on the incident side. The polarized light axes of the two polarizing plates 21, 22 are parallel or perpendicular. A system arranged for making two polarized light axes parallel to each other is said to be in a normally black (NB) mode. A system arranged for making two polarized light axes orthogonal with respect to each other is said to be in a normally white (NW) mode. In a projection displaying apparatus, the NW mode is often used. In the NB mode, chromaticity changes are caused to be near black display, which is too conspicuous. Such a problem does not occur in the NW mode. In the NW mode, the chromaticity changes are caused to be near the white display, which is not too conspicuous.
An operation of the TN liquid crystal panel in the NW mode will be described hereinafter. When natural light is inputted into the polarizing plate 21 on the incident side, linearly polarized light is outputted, and the linearly polarized light is inputted into the liquid crystal cell 20. When a voltage is not applied to the liquid crystal layer 19, the polarization wave face of the linearly polarized light is rotated by 90.degree. by the liquid crystal layer 19 so that it passes through the polarizing plate 22 on the outgoing side so as to effect a white displaying operation. When sufficient voltage is applied to the liquid crystal layer 19, rotation of the light by the liquid crystal layer 19 is removed. As the light passes through the liquid crystal layer 19 without the rotation of the polarization face of the linearly polarized light, it does not pass through the polarizing plate 22 on the outgoing side, so as to effect a black displaying operation.
The TN liquid crystal panel 14 can form optical images as changes in transmission index by controlling the applied voltage. When the TN liquid crystal panel is used as a light valve, the image display of a larger picture face can be effected if the picture face size of the liquid crystal panel is smaller.
Recently, in order to use the brightness and resolution of the projected image, three liquid crystal panels have been mainly used for red, green, and blue colors, respectively. A front system where a projector and a screen are separated from each other (for example, Japanese Patent Laid-Open Publication No. 62-133424), and a rear system where a transmission type of screen is mounted in the front portion of a cabinet and all the optical components are accommodated within the cabinet (for example, U.S. Pat. No. 5,042,929) have been proposed. Both the systems have received public attention because they have resulted in more compact arrangements.
Components of the optical head of the front system are shown in FIG. 19. Light coming from a light source 31 is inputted into a chrominance resolution optical head composed of dichroic mirrors 32, 33 and a flat mirror 34 and are resolved into lights of the three (red, green and blue) primary colors. The primary color lights are respectively transmitted through field lenses 35, 36 and 37, and thereafter, are inputted into liquid crystal panels 38, 39 and 40. Optical images are formed on the liquid crystal panels 38, 39 and 40 as changes in transmission index in accordance with image signals. Outgoing lights from the liquid crystal panels 38, 39 and 40 are composed into one light by a chrominance composition optical head composed of dichroic mirrors 41, 42 and a flat mirror 43. The composed light is inputted into a projection lens 44. Optical images on three liquid crystal panels 38, 39 and 40 are enlarged, and projected onto a screen by a projection lens 44.
Components of an optical head of the rear system are shown in FIG. 20. A transmission type screen 52 is disposed in the upper portion on the front side of a cabinet 51. A projector is disposed in a rear lower position. A flat mirror 54 is disposed in a forward lower position. A flat mirror 55 is disposed rearwardly of the screen 52. A projection distance (light path length from a projection lens to the center of the screen) is shortened and the projector can be made smaller in size so that the cabinet 51 can be made more compact. The screen 52 is composed of a flannel lens combined with a renticular plate.
The projector is composed of a light source 56, a liquid crystal panel 57 and a projection lens 58. Lights coming from the light source 56 are bent in direction by a mirror 53 after passing through the liquid crystal panel 57 and are inputted into the projection lens 58. Optical lights as changes in the transmission index are formed on the liquid crystal panel 57 in accordance with the image signals. The optical images on the liquid crystal panel 57 are enlarged, and are projected onto the screen 52 by the projection lens 58.
The relationship between a transmission index T and an incident angle .theta. when a maximum driving voltage is applied upon the TN crystal liquid panel of the NW mode so as to make parallel lights incident is shown in FIG. 21. The transmission index T becomes minimum when an incident angle .theta. is a certain angle .theta.o. Assume that an incident angle .theta.o where the transmission index becomes minimum is called an optimum incident angle. .theta.o.noteq.0. A transmission index at .theta.=.theta.o is smaller than a transmission index at .theta.=0.degree.. Normally when a liquid crystal layer thickness of the liquid crystal panel is 4 through 5 .mu.m, and an applied voltage is 5 through 6 V, an optimum incident angle .theta.o is 3.degree. through 5.degree.. It is better for the contrast of the display images to make light incident from a slightly oblique direction, instead of making the lights incident perpendicular to the liquid crystal cell.
A projection displaying apparatus using the fact that lights incident obliquely to the liquid crystal panel are better in contrast has been proposed (for example, Japanese Patent Laid-Open Publication No. 63-73782 and Japanese Patent Laid-Open Publication No. 3-71110). The components of this example are shown in FIG. 22. Although the liquid crystal layer 19 is perpendicular to the optical axis 61 of the projection lens 15, the picture center 62 of the liquid crystal panel 14 is deviated from the optical axis 61 of the projection lens 15. The optical axis of the light source 11 is also made parallel to the optical axis 61 so that an angle, made by a straight line for connecting the picture face center 62 of the liquid crystal panel 14 with the eye center 63 of the projection lens 15, and the optical axis 61 of the projection lens 15, may be at an optimum incident angle. In the construction shown in FIG. 22, contrast in the picture center is better than that for the construction shown in FIG. 18.
A projection display of a construction shown in FIG. 22 is required to make the maximum picture angle of the projection lens larger as compared with the construction shown in FIG. 18. The illumination intensity of an area far from the projection lens among the projection images becomes lower. This fact becomes conspicuous as the maximum picture angle of the projection lens of the basic construction shown in FIG. 18 becomes larger. It becomes an especially serious problem in the rear system. The maximum picture angle of the projection lens 58 is required to be much larger so as to make the lights incident obliquely to the liquid crystal panel 57 for improvement in the contrast in the construction shown in, for example, FIG. 20. Thus, the intensity of illumination in an area far from the projection lens 58 on the screen 52 becomes considerably lower as compared with the illumination intensity at the center of the picture. If the surrounding illumination intensity ratio is made larger, the projection lens 58 will become larger and longer, and it will be difficult to make the cabinet 51 compact.
It has been considered to raise a driving voltage to be applied upon the liquid crystal layer to 10 V or more as a method of improving the contrast of the projection image. This method requires that the power voltage of a drive IC be raised. The temperature of the liquid crystal panel will therefore be raised so as to increase the generation of caloric value (heat) of the drive IC, and thus the picture quality of the display images will be undesirably degraded.
A view finder using a CRT was mounted on the conventional video camera. A video camera which is smaller in size and lighter in weight is desired. Therefore, a view finder using a TN liquid crystal panel was used. However, the contrast of the display images was inferior. The cause is the same as in the projection display apparatus using a liquid crystal panel. Although the contrast is better when the display image of the liquid crystal panel is seen from the oblique direction, it is hard for a user to see.
As described hereinabove, a liquid crystal displaying apparatus where the display images become higher in contrast can be realized when the lights are inputted at a perpendicular incidence.